1. Field of the Invention
This invention relates to circulation control aerofoils and particularly to such aerofoils for use in the rotor blades of helicopter main sustaining rotors.
2. Description of the Prior Art
In a circulation control helicopter sustaining rotor, pressurised air is supplied through control valve means, usually in a rotor head, to the hollow interior of each of a plurality of rotor blades. The pressurised air is forced out of spanwise extending slots located adjacent the blade trailing edge and sometimes adjacent both the trailing and leading edges.
Circulation control of a helicopter rotor has been proposed principally to vary a blade maximum lift coefficient for thrust control purposes and particularly to increase this coefficient to mitigate the problem of retreating blade stall. In general, a greater amount of blowing airflow is required to be ejected from a retreating blade than from an advancing blade and mechanical control valve means have therefore been proposed to cyclically modulate the amount of ejected airflow to achieve cyclic lift control as the blade rotates around the rotor disc, for the purposes of trim and control, as well as collectively modulating the amount of the ejected airflow to achieve collective control of the rotor thrust normal to the rotor plane.
An example of a prior art mechanical control valve means is disclosed in U.S. Pat. no. 4,799,859. That device is complex, heavy and may be unreliable in operation.
In seeking to maximise the thrust from a helicopter main sustaining rotor (or to minimise the power required to generate a given thrust) it is known that there is an optimum distribution of lift (local lift coefficient) at different stations throughout the blade span. In addition, the optimal radial distribution of lift varies according to the azimuthal position of the rotor blade in the rotor disc, and with the helicopter flight conditions e.g. speed, altitude, manoeuvre etc.
Hence, to maximise the performance of a helicopter sustaining rotor it is, ideally, required to vary the local lift coefficient at different stations along the span of each blade, and as the blade rotates around the rotor disc.
The inventors have realised the potential of circulation control for achieving such a variation of local lift coefficient in a helicopter rotor. However, they realised also that the requirements may not optimally be met using the mechanical control valve arrangements and air distribution techniques of the prior art, since the need to introduce variable spanwise control along individual blades would further complicate such existing complex systems and add to the existing problems.
A primary objective of this invention therefore is to provide a circulation control aerofoil that is simpler and better adapted to use in a helicopter rotor than the prior art. Another objective is to provide such an aerofoil having at least the potential to vary the local lift coefficient at different stations along the aerofoil span.